Method to deposit an impermeable film on porous low-k dielectric film

ABSTRACT

A method for improving the adhesion of an impermeable film on a porous low-k dielectric film in an interconnect structure is disclosed. The method provides an in-situ annealing step before the deposition of the impermeable film to release the volatile trapped molecules such as water, alcohol, HCl, and HF vapor, inside the pores of the porous low-k dielectric film. The method also provides an in-situ deposition step of the impermeable film right after the deposition of the porous low dielectric film without exposure to an atmosphere containing trappable molecules. The method further provides an in-situ deposition step of the impermeable film right after the removal a portion of the porous low-k dielectric film without exposure to an atmosphere containing trappable molecules. By the removal of all trapped molecules inside the porous low-k dielectric film, the adhesion between the deposited impermeable film and the low-k dielectric film is improved. This method is applicable to many porous low-k dielectric films such as porous hydrosilsesquioxane or porous methyl silsesquioxane, porous silica structures such as aerogel, low temperature deposited silicon carbon films, low temperature deposited Si—O—C films, and methyl doped porous silica.

REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 10/360,133, filed Feb. 4, 2003, (Atty. Docket No.TEGL-01187US0), which is incorporated herein by reference.

FIELD OF THE INVENTION

This invention relates generally to integrated circuit processes andfabrication, and more particularly, to a method to deposit animpermeable film on a porous low-k dielectric film.

BACKGROUND OF THE INVENTION

The demand for progressively smaller, less expensive, and more powerfulelectronic products creates a need for smaller geometry integratedcircuits (ICs) and larger substrates. It also creates a demand fordenser packaging of circuits onto IC substrates. The desire for smallergeometry IC circuits requires that the dimensions of interconnectionsbetween the components and the dielectric layers be as small aspossible. Therefore, recent research continues to focus on the use oflow resistance materials (e.g., copper) in conjunction with insulatingmaterials with low dielectric constant (k) between the metal lines.

The use of low resistance materials is needed because of the reductionin the cross sectional area of via interconnects and connecting lines.The conductivity of interconnects is reduced as the surface area ofinterconnects is reduced, and the resulting increase in interconnectresistivity has become an obstacle in IC design. Conductors having highresistivity create conduction paths with high impedance and largepropagation delays. These problems result in unreliable signal timing,unreliable voltage levels, and lengthy signal delays between componentsin the IC. Propagation discontinuities also result from intersectingconduction surfaces that are poorly connected and from the joining ofconductors having highly different resistivity characteristics.

There is a need for low resistivity interconnects and vias that have theability to withstand volatile process environments. Aluminum andtungsten metals are often used in the production of integrated circuitsfor making interconnections or vias between electrically active areas.These metals have been used for a long time in the productionenvironment because the processing technologies for these metals wereavailable. Much experience and expertise with these metals have beenacquired.

Copper is a natural choice to replace aluminum in the effort to reducethe size of lines and vias in an electrical circuit. The conductivity ofcopper is approximately twice that of aluminum and over three times thatof tungsten. As a result, the same current can be carried through acopper line having half the width of an aluminum line.

However, there have been problems associated with the use of copper inIC processing. Copper poisons the active area of silicon devices,creating unpredictable responses. Copper also diffuses easily throughmany materials used in IC processes and, therefore, care must be takento keep copper from migrating.

Various means have been suggested to deal with the problem of copperdiffusion into integrated circuit materials. Several materials,including metals and metal alloys, have been suggested for use asbarriers to prevent copper diffusion. The typical conductive diffusionbarrier materials are TiN, TaN and WN. Addition of silicon into thesematerials to create TiSiN, TaSiN, and WSiN, respectively, could offerimprovements in the diffusion barrier properties. Silicon nitride hasbeen the best non-conductive diffusion barrier material so far.

Diffusion barrier materials could be deposited by the chemical vapordeposition technique. For example, in the case of TiN CVD deposition, aprecursor that contains Ti and optionally nitrogen is used. Theprecursor decomposes at the selected surfaces, and the decomposedelements react together to form a TiN layer on the selected surfaces.Reaction by-products (i.e., products produced by the precursordecomposition and the following reactions that do not deposited on theselected surfaces) are often volatile and are exhausted away.

Of equal importance with the use of low resistance materials ininterconnecting lines is the introduction of low dielectric constantmaterials (low-k dielectrics) for insulating between the interconnectinglines. Low k dielectrics are insulating dielectric materials thatexhibit dielectric constants that are less than those of conventional ICdielectric materials such as silicon dioxide (k value of about 4),silicon nitride (k value of about 7), and silicon oxynitride (k value ofabout between 4 and 7).

Various low-k dielectrics have been introduced including fluorine dopedsilicon dioxide (k value of about 3-3.6), carbon doped silicon dioxide(k value of about 2.5-3.3), fluorinate carbon (k value of about2.5-3.6), and organic materials such as parylene (k value of about3.8-3.6) and polyimide (k value of about 3-3.7). Some of these materialshave been successfully incorporated into the IC fabrication processes,but others have not been because of various difficulties involved withthe integration. The low k dielectrics can be deposited by CVD orspin-on techniques.

Further research is focusing on porous low-k dielectrics because oftheir potential lower dielectric constants (2-3). Examples of porous lowdielectric materials are porous hydrosilsesquioxanc or porous methylsilsesquioxane, porous silica structures such as aerogel, lowtemperature deposited silicon carbon films, low temperature depositedSi—O—C films, and methyl doped porous silica.

The use of porous low-k dielectrics presents significant integrationproblems such as low mechanical strength, poor dimensional stability,poor temperature stability, high moisture absorption, permeation, pooradhesion, large thermal expansion coefficient, and unstable stresslevel.

Of the various problems associated with porous low-k dielectrics, thetrapping of small molecules in porous low-k dielectrics is one that isrecognized in IC processes. U.S. Pat. No. 6,417,118 to Hu et al.discloses a method to prevent further absorption of moisture into aporous low-k dielectric film by treating the porous film with a reactivesolution to convert the porous low-k dielectric surface from thehydrophillic state (attracting moisture) to the hydrophobic state(repelling moisture) after all the trapped moisture was removed by lowtemperature annealing. U.S. Pat. No. 6,486,061 to Xia et al. discloses amethod for providing a dielectric film having enhanced adhesion andstability that uses post deposition treatment that densifies the film ina reducing environment such as NH₃ or H₂. By post deposition annealingin NH₃ or H₂, Xia et al. found that the dielectric film becomes moremoisture resistant and retains a low value of dielectric constant evenwhen exposed to the ambient for a week.

The integration of porous low-k dielectrics remains a problem. Even withtreatments of low-k dielectric films, the adhesion of the subsequentfilm, such as a diffusion barrier film for copper interconnect, remainsproblematic. Since the subsequently deposited films are oftenimpermeable to the trapped molecules such as moisture, alcohol vapor,HCl vapor, and HF vapor, the release of these trapped molecules cancause delamination that leads to device failure.

SUMMARY OF THE INVENTION

Accordingly, a method of improving the adhesion of a subsequentlydeposited impermeable film onto a porous low-k dielectric film isprovided.

The disclosed method basically ensures that the porous low-k dielectricfilm will not be exposed to an atmosphere containing trappable moleculessuch as moisture before depositing a subsequent impermeable film such asa conductive diffusion barrier (TiN, TiSiN, TaN, TaSiN, WN_(x), WSiN) ora dielectric diffusion barrier (SiC, Si₃N₄).

The prior art discloses various methods of treating porous low-kdielectric film to improve the amount of moisture absorption. But ourresearch indicates that these methods can only reduce the amount ofmoisture absorption, but cannot eliminate it. Xia et al. discloses thattheir treatment of annealing in a reducing environment is effective inincreasing the moisture resistant property and in retaining thedielectric constant value after exposing to air for one week but issilent on the adhesion of subsequent impermeable film such as adiffusion barrier. Our research indicates that this treatment is not atall effective in improving the adhesion of TiN on the porous low-kdielectric film after exposing to air even for a few hours. Afterextensive evaluations of various treatments, the only effectivetreatment we found that improves the adhesion of the subsequentimpermeable film such as a diffusion barrier to the porous low-kdielectric film is by not exposing the porous low-k dielectric film to amoisture containing ambient. Moisture will be trapped inside the poreswhen a porous low-k dielectric film is exposed to air, and without theremoval of the trapped moisture, the adhesion of the subsequentlydeposited impermeable film to the porous low-k dielectric film will begradually degraded as a result of the release of the trapped moisture.

In the case where the porous low-k dielectric film already containsmoisture, the method provides additional steps of removing the moisturetrapped in the pores of low-k dielectric films before in-situ depositingthe impermeable film.

In the first preferred embodiment, the method comprises two steps:

a. annealing the porous low k dielectric film to remove the volatilemolecules trapped inside the pores of porous low-k dielectric films; and

b. depositing an impermeable film onto the porous low-k dielectric filmswithout exposing the porous low-k dielectric film to an atmospherecontaining trappable molecules.

The first embodiment addresses the situation where the porous low-kdielectric film has been exposed to air and therefore has much moisturetrapped inside the pores. To successfully deposit an adheringimpermeable film onto the porous low-k dielectric film, the moistureneeds to be removed and the impermeable film is deposited without anyexposure of the porous low-k dielectric films to an atmospherecontaining trappable molecules.

The most common volatile molecules trapped inside the pores of porouslow-k dielectric film are moisture. Other volatile molecules arealcohol, HCl, and HF. The volatile molecules are the molecules that arein their gaseous state at room temperature or higher temperatures. Thevolatile molecules can be organic or inorganic materials.

The anneal temperature can be between 50° C. to 500° C. Highertemperatures can drive out moisture in a shorter time, but the highertemperature can damage the porous low-k dielectric films. The annealtime can be between 10 seconds to 2 hours, depending on the annealtemperature and the state of the porous low-k dielectric films. Aresistive or a radiative heater can be used for the anneal process. Theanneal process can be done in an inert gas ambient such as helium,argon, or nitrogen. The anneal process can also be done in a reactiveambient such as in NH₃ or hydrogen. The anneal process can also be donein a sub-atmospheric pressure ambient, typically in a pressure of a fewTorr or a few milliTorr.

The anneal step and the deposition step can be processed in the samechamber or in different chambers. In the case of different chambers, themethod provides for an additional step of transferring the workpiececontaining the porous low-k dielectric film from the anneal chamber tothe deposition chamber. The transfer is done in an ambient notcontaining any trappable molecules, such as an inert gas ambient(helium, argon, or nitrogen), or a reactive ambient (NH₃ or hydrogen),or sub-atmospheric pressure ambient, typically in a pressure of a fewTorr or a few milliTorr. There can be a transfer chamber to temporarilyhouse the work piece for the transfer. The anneal chamber or thedeposition chamber can be a single work piece processing chamber, or amultiple work piece processing chamber. The anneal chamber and thedeposition chamber can both be a single work piece processing chamber,or can both be a multiple work piece processing chamber, or can be anycombination. The choice of chamber can in part be dependent on thedesired throughput. If the anneal step is much longer than thedeposition step, it is advantageous to use a multiple work pieceannealing chamber than to use a single work piece deposition chamber.

The porous low-k dielectric film can be porous hydrosilsesquioxane(porous HSQ) or porous methyl silsesquioxane (porous MSQ), porous silicastructures such as aerogel, low temperature deposited silicon carbonfilms, low temperature deposited Si—O—C films, and methyl doped poroussilica. The porous low-k dielectric films can have a passivation layeron top of the porous low-k dielectric films. The impermeable film can beTiN, TaN, WN_(x), TiSiN, TaSiN, WSiN, SiO₂, Si₃N₄, silicon carbide,metal films such as copper, tungsten, aluminum, or a Si film such aspolysilicon, and amorphous silicon. The impermeable film can bedeposited by CVD (chemical vapor deposition) technique, NLD (nanolayerdeposition) technique, ALD (atomic layer deposition) technique, orsputtering technique.

In the second preferred embodiment, the method comprises two steps:

a. depositing the porous low-k dielectric films; and

b. depositing an impermeable film onto the porous low-k dielectric filmswithout exposing the porous low-k dielectric film to an atmospherecontaining trappable molecules.

The second embodiment addresses the situation where the porous low-kdielectric film has not been exposed to the air ambient and thereforehas no moisture trapped inside the porous low-k dielectric film. Tosuccessfully deposit an adherence impermeable film onto the porous low-kdielectric film, the impermeable film is deposited without any exposureof the porous low-k dielectric film to an atmosphere containingtrappable molecules.

The porous low-k dielectric film can be deposited by spin-on techniqueor CVD technique. The deposition technique can comprise the actualdeposition step plus any other steps necessary, such as a film curingstep, to ensure a useable porous low-k dielectric film. The method cancomprise a further step, after the deposition of the porous low-kdielectric film, of depositing a passivation layer on top of the porouslow-k dielectric film.

In the third preferred embodiment, the method comprises two steps:

a. removing a portion of the porous low-k dielectric film; and

b. depositing an impermeable film onto the porous low k dielectric filmwithout exposing the porous low-k dielectric film to an atmospherecontaining trappable molecules.

The third embodiment addresses the situation where the porous lowdielectric film has not been exposed to the air ambient, or has beenannealed to remove all moisture, and therefore has no moisture trappedinside the porous low k dielectric film. However, before the depositionof the impermeable film such as a diffusion barrier layer, the porouslow-k dielectric film will need to undergo a patterning step. Thepatterning step will remove a select portion of the porous low-kdielectric film. The removal of a selected portion of the porous lowdielectric film will expose the porous low dielectric film even with apassivation layer. To successfully deposit an adhering impermeable filmonto the porous low k dielectric film, the impermeable film is depositedwithout any exposure of the porous low-k dielectric film to anatmosphere containing trappable molecules after the removal of a portionof the porous low-k dielectric film.

The porous low-k dielectric film can have a passivation layer afterdeposition to protect the top surface. The removal step can be a wetetch step or a plasma enhanced dry etch step. The method can furthercomprise an additional step in between these two steps. The additionalstep can be a cleaning step to clean the porous low-k dielectric filmand to prepare the porous low-k dielectric film before the deposition ofthe impermeable film. The additional step can be a photoresist strippingstep in the case the porous low-k dielectric film has underwent aphotolithography step of patterning which uses photoresist as a methodof patterning. The additional step can be an anneal step to drive outall possible moisture or any trapped molecules inside the pore of porouslow-k dielectric film before the impermeable film deposition step.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flowchart showing the steps in the first embodiment of themethod of improving the adhesion of an impermeable film onto a porouslow dielectric film in an integrated circuit processing.

FIG. 2 is a flowchart showing the steps in the second embodiment of themethod of improving the adhesion of an impermeable film onto a porouslow-k dielectric film in an integrated circuit processing.

FIG. 3 is a flowchart showing the steps in the third embodiment of themethod of improving the adhesion of an impermeable film onto a porouslow-k dielectric film in an integrated circuit processing.

FIGS. 4 a-4 f show the schematics of a typical integrated processing.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 is a flowchart showing the steps in the first embodiment of themethod of improving the adhesion of an impermeable film onto a porouslow dielectric film in an integrated circuit processing. Step 11 selectsan integrated circuit that includes a porous low-k dielectric film. Theporous low-k dielectric film has been exposed to an atmospherecontaining trappable organic molecules such as the air ambientcontaining moisture. Step 12 shows the current invention adhesionimprovement process comprising two steps: step 14 and step 15. Step 14provides the annealing process to remove all trappable organic moleculesinside the porous low-k dielectric film. Then step 15 provides thedeposition of the impermeable film on top of the porous low dielectricfilm without exposing the porous low-k dielectric film to an ambientcontaining trappable organic molecules. Step 13 provides the rest of theintegrated circuit processing such as interconnect and passivation. Inorder not to expose the porous low-k dielectric film to an ambientcontaining trappable organic molecules, the anneal step 14 and thedeposition step 15 can be processed in the same process chamber, or in acluster system. A cluster system can have four to six process chambersconnected to a transfer chamber with a robotic system capable oftransferring the wafers from one process chamber to another processchamber. The process chambers and the transfer chamber are typicallyunder vacuum with a pressure of milliTorr or microTorr. The chambers inthe cluster system are well maintained to ensure minimum amounts ofmoisture and organic molecules.

FIG. 2 is a flowchart showing the steps in the second embodiment of themethod of improving the adhesion of an impermeable film onto a porouslow-k dielectric film in an integrated circuit processing. Step 21selects integrated circuit. Step 22 shows the current invention adhesionimprovement process comprising three steps: step 24, step 26 and step25. Step 24 provides the deposition of the porous low-k dielectric film.Then step 26 provides all other processes or a transfer process withoutexposing the porous low-k dielectric film to an atmosphere containingtrappable organic molecules. Finally, step 25 provides the deposition ofthe impermeable film on top of the porous low-k dielectric film withoutexposing the porous low dielectric film to an atmosphere containingtrappable organic molecules. Step 23 provides the rest of the integratedcircuit processing such as interconnect and passivation. Step 26 is anoptional step such as the deposition of a passivation or a cap layer ontop of the porous low-k dielectric film, or the etching or patterning ofthe porous low-k dielectric film before the deposition of theimpermeable film. Typically, the deposition of the porous low dielectricfilm and the deposition of the impermeable film occurred in two separateprocessing chamber connected to a cluster system. Since a cluster systemcan have four to six process chambers, the other process chambers can beused for optional processes without exposing the porous low-k dielectricfilm to undesirable ambient.

FIG. 3 is a flowchart showing the steps in the third embodiment of themethod of improving the adhesion of an impermeable film onto a porouslow dielectric film in an integrated circuit processing. Step 31 selectsintegrated circuit including a porous low-k dielectric film. Step 32shows the current invention adhesion improvement process comprisingthree steps: step 34, step 36 and step 35. Step 34 provides the removalof a portion of the porous low-k dielectric film, typically by a plasmaetch process. Then step 36 provides all other processes or a transferprocess without exposing the porous low-k dielectric film to anatmosphere containing trappable organic molecules. Finally, step 35provides the deposition of the impermeable film on top of the porous lowk dielectric film without exposing the porous low dielectric film to anatmosphere containing trappable organic molecules. Step 33 provides therest of the integrated circuit processing such as interconnect andpassivation. Step 36 is an optional step such as the cleaning orphotoresist stripping after the removal step 34 of a portion of theporous low-k dielectric film. The removal step 34 typically requires theuse of a photoresist deposition for the pattern transfer, therefore step36 provides an intermediate step of removing the residue photoresist,and the cleaning of the porous low-k dielectric film before thedeposition of the impermeable film. Typically, the etching of the porouslow-k dielectric film and the deposition of the impermeable film areoccurred in two separate processing chamber connected to a clustersystem. Since a cluster system can have four to six process chambers,the other process chambers can be used for optional processes such asresist stripping or cleaning or even annealing without exposing theporous low dielectric film to undesirable ambient.

FIGS. 4 a through 4F show the schematic of a typical integratedprocessing incorporating the present invention. FIG. 4 a shows a typicalinterconnect underlayer. The underlayer comprises a bottom dielectriclayer 40 with a bottom conducting line 42 and a top dielectric layer 44.The bottom conducting layer 42 is covered by a diffusion barrier layer41. Layer 43 is a top diffusion barrier for the bottom conducting layer42 and also serves as an etch stop layer. Layer 45 is optional andserves as a cap layer or a passivation layer for the dielectric layer44. The dielectric layers 40 and 44 can be porous low-k dielectric filmsto reduce the propagation delay.

FIG. 4 b shows the patterning step of the porous low-k dielectric film44. A photoresist film 46 is coated on the cap layer 45 (or on theporous low dielectric film 44 if there is no cap layer 45). Thephotoresist is then exposed with a pattern mask and then the exposedphotoresist is developed and removed. The photoresist now contains thepattern from the mask.

FIG. 4 c shows the transfer of the photoresist pattern onto the porouslow-k dielectric film by a plasma etch process. The photoresist protectsthe underlayer and the etch process only etches the exposed area asshown in FIG. 4 c.

FIG. 4 d shows the next step of removing the residue photoresist andcleaning of the porous low-k dielectric film 44.

FIG. 4 e shows the step of depositing an impermeable film 47 such as adiffusion barrier. And FIG. 4 f shows the step of depositing the metalconduction line 48. Typical diffusion barriers for semiconductorinterconnect are TiN, TiSiN, TaN, TaSiN, WN, and WSiN, for copperinterconnect and aluminum interconnect.

1. A method to improve the adhesion of an impermeable film on a porouslow dielectric film that is deposited on a work piece, said methodcomprising the steps of: removing a portion of the porous low-kdielectric film, wherein the porous low-k dielectric film does notcontain any trappable molecules; and depositing an impermeable film onthe porous low-k dielectric film without exposing the porous low-kdielectric film to an atmosphere containing trappable molecules.
 2. Amethod as in claim 1 wherein the removal step is by a plasma etching. 3.A method as in claim 1 further comprising an intermediate step ofcleaning the porous low-k dielectric film before depositing of theimpermeable film.
 4. A method as in claim 3 wherein the cleaning stepcomprises a stripping photoresist.
 5. A method as in claim 1 furthercomprising an intermediate step of annealing the porous low-k dielectricfilm by heating the porous low dielectric film for a sufficient lengthof time to remove volatile molecules that are trapped inside the porouslow-k dielectric film, before the deposition of the impermeable film. 6.A method as in claim 5 wherein the volatile molecules are selected fromthe group consisting of moisture, alcohol vapor, HCl vapor, and HFvapor.
 7. A method as in claim 5 wherein the temperature of the annealprocess is between 50° C. and 500° C.
 8. A method as in claim 5 whereinthe anneal time is between 10 seconds and 2 hours.
 9. A method as inclaim 5 wherein the anneal process is performed by resistive heater orradiative heater.
 10. A method as in claim 5 wherein annealing occurs inan ambient that is selected from the group consisting of nitrogen, andinert gases.
 11. A method as in claim 1 wherein the porous material isselected from the group consisting of porous MSQ, porous HSQ, poroussilica structures, low temperature deposited silicon carbon films, lowtemperature deposited Si—O—C films, and methyl doped porous silica. 12.A method as in claim 1 wherein the impermeable film is formed ofmaterial that is selected from the group consisting of TiN, TaN, WN,TiSiN, TaSiN, WSiN, SiO₂, Si₃N₄, metal, and Si.
 13. A method as in claim1 wherein the deposition method to deposit the impermeable film isselected from the group consisting of a CVD, ALD, and nanolayerdeposition NLD sputtering techniques.
 14. A method as in claim 1 whereinthe porous low dielectric film is deposited on an integrated circuit.15. A method as in claim 5 wherein the anneal process removesessentially all volatile molecules including organic material andmoisture from the porous low-k dielectric film.
 16. A method as in claim15 wherein the porous material is selected from the group consisting ofporous MSQ, porous HSQ, porous silica structures, low temperaturedeposited silicon carbon films, low temperature deposited Si—O—C films,and methyl doped porous silica.
 17. A method as in claim 15 wherein theimpermeable film is formed of material that is selected from the groupconsisting of TiN, TaN, WN, TiSiN, TaSiN, WSiN, SiO₂, Si₃N₄, metal, andSi.